/**

* Definition for a Node.

* public class Node {

* public var val: Int

* public var neighbors: [Node?]

* public init(_ val: Int) {

* self.val = val

* self.neighbors = []

* }

* }

*/

func cloneGraph(_ node:Node?)->Node?{

guardlet node= nodeelse{returnnil}

// check if cache exists

ifmapping[node]!=nil{

returnmapping[node]!

// otherwise, create a node, cache it, recurse for children

letnewNode=Node(node.val)

mapping[node]= newNode

node.neighbors.forEach{ neighborin

newNode.neighbors.append(cloneGraph(neighbor))

return newNode